The bacterial pathogen, Mycobacterium tuberculosis, is responsible for 8 million new cases of tuberculosis and 2.9 million deaths per year, worldwide. The host immune responses necessary for protection against disease are not clearly defined. Cell-mediated immunity is required for protection, and recent work has confirmed a role for both CD4 and CD8 T cells in the immune response to this pathogen. Here, experiments designed to examine the exact nature of the contribution of CD8 T cells to the protective immune response are proposed. We have outlined a strategy for determining the actual function of CD8 T cells, either as cytotoxic T cells or IFN-gamma producing cells, or both, in M. tuberculosis infection. This involves testing for the presence of mycobacterial specific CD8 CTLs in various mice, establishing CTL lines, and testing these lines for protective capacity in vivo. A systematic approach to this problem is proposed, including the use of various alternative target cells and sources for CD8 T cells. As an alternative hypothesis, the role of IFN-gamma produced by CD8 T cells will be examined. Using in vitro methods and immunological complementation in gene-disrupted mice, we will determine the role of CD8 T cells in protection against tuberculosis in mice. These studies on the actual role of CD8 T cells will increase our understanding of the host immune responses necessary for protection against M. tuberculosis. Information obtained from the results of the proposed experiments will be of value in designing new immunization strategies for tuberculosis and may suggest potential pathogenic mechanisms by which mycobacteria evade the necessary immune responses, and unveil possible virulence factors for further study.